The present invention relates to a scanning optical system for forming a laser beam scanning on a scan target surface.
In general, a scanning optical system is employed in, for example, a laser beam printer, a digital copying machine, a laser fax machine and a laser plotter. In such an apparatus, the scanning optical system is used to form a modulated beam scanning on the scan target surface (e.g., a photoconductive drum).
The scanning optical system is generally provided with a polygonal mirror which dynamically deflects laser beams on/off modulated according to image data, and an imaging optical system which converges the deflected laser beam on the scan target surface to form a beam spot scanning in a main scanning direction at a constant speed. Consequently, a two dimensional latent image is formed on the scan target surface.
Hereafter, a direction in which the beam spot is scanned on the scan target surface is referred to as the main scanning direction, and a direction perpendicular to the main scanning direction is referred to as an auxiliary scanning direction. In the following description, the shape of optical elements, directions of power of the optical elements and the like are described with reference to the main and auxiliary scanning directions on the surface to be scanned. That is, if an optical element is described to have a refractive power in the main scanning direction, the power affects the beam in the main scanning direction on the scan target surface regardless of the orientation of the element.
To remove ghost images due to undesired reflections is important design requirements of the scanning optical system. As mentioned below, such undesired reflections are caused by lens surfaces of the imaging optical system.
Part of a beam is reflected by a lens surface of the imaging optical system when the beam passes through the lens surface. The part of the beam (i.e., a reflected beam) proceeds in a direction defined depending on an incident angle a beam axis of the beam incident on the lens surface and on a shape of the lens surface. If the reflected beam impinges on one of reflective surface of the polygonal mirror, the reflected beam is reflected by the polygonal mirror again.
If a reflective surface on which the reflected beam impinges is coincides with a reflective surface currently deflecting the beam from a light source, the reflected beam twice deflected by the polygonal mirror does not enter the imaging optical system because the twice deflected beam proceeds toward the light source. That is, in this case, the reflected beam does not impinge on the scan target surface and therefore imaging quality is not deteriorated.
On the other hand, if the reflected beam impinges on a reflective surface neighboring a reflective surface currently deflecting the beam from the light source, the reflected beam twice deflected by the polygonal mirror may enter the imaging optical system. Therefore, there is a possibility that the photoconductive drum is exposed to undesired beams. If the photoconductive drum is exposed to the undesired beams, imaging quality is deteriorated. In this specification, such an undesired beam (i.e., the reflected beam) which impinges the polygonal mirror again is referred to as a ghost beam.
FIG. 9 is a top view of a conventional scanning optical system 90 used for a monochrome laser beam printer. FIG. 10 is a side view of the scanning optical system 90. As shown in FIG. 9, the scanning optical system 90 includes a light source 46, a polygonal mirror 40 and an fθ lens 45. The fθ lens 45 has a scanning lens 41 and a compensation lens 42.
The scanning lens 41 has power mainly in the main scanning direction and is located on a polygonal mirror side. The compensation lens 42 has the function of compensating for curvature of field and an fθ error and is located on a scan target surface side.
An optical path of a ghost beam is also indicated in FIGS. 9 and 10. Each of FIGS. 9 and 10 shows a situation where part of a beam passing through the compensation lens 42 is reflected by a lens surface 42a and impinges on a reflective surface 40b adjoining to a reflective surface 40a currently deflecting the beam from the light source 46. Further, the beam deflected by the reflective surface 40b passes through the fθ lens 45 and impinges on the scan target surface S. Consequently, a photoconductive material placed on the scan target surface S is exposed to the beam (ghost beam).
The occurrence of the ghost beam can be prevented by reducing reflectivity of lens surfaces of the fθ lens 45 to zero. However, a lens having very low reflectivity is expensive. The reason is that the lower reflectivity becomes, the higher the number of layers of coatings to be formed on a lens surface becomes. In addition, to reduce reflectivity of a lens surface to completely zero by use of the coatings is impossible.
Japanese Provisional Publication No. HEI 5-346553 discloses a scanning optical system configured to prevent the occurrence of the ghost beam. In this scanning optical system, a lens having a lens surface which has a possibility of reflecting incident beam (i.e., making the ghost beam) is inclined by a predetermined angle in an auxiliary scanning plane (which is defined as a plane including a rotational axis of a polygonal mirror and an optical axis of a scanning lens).
Japanese Provisional Publication No. HEI 7-230051 discloses another scanning optical system configured to prevent the occurrence of ghost beams. In the scanning optical system disclosed in this publication, a lens surface of a scanning lens system is decentered in the auxiliary scanning direction to prevent the occurrence of the ghost beam. A bow (i.e., a curve of a scanning line) caused by the decentering of the lens surface is compensated by decentering another lens surface of the scanning lens system in the auxiliary scanning direction.
Considering the configuration to prevent the ghost beam and the bow using the example of FIG. 9, even if the compensation lens 42 having a lens surface generating the ghost beam is decentered for preventing the ghost image, a bow caused by the decentering of the compensation lens 42 can be compensated by decentering the scanning lens 41.
Configurations to prevent the occurrence of the ghost beam described in the above two publications are useful for a scanning optical system for an apparatus for forming monochrome images such as a monochrome laser beam printer because such an apparatus requires only one scanning beam.
However, the configurations to prevent the occurrence of ghost beams described in the above publications can not be applied to a tandem-type scanning optical system used for an apparatus for generating color images such as a color laser beam printer.
FIG. 11 is a perspective view of the tandem-type scanning optical system 110. As shown in FIG. 11, the tandem-type scanning optical system 110 includes a light source 56 which emits a plurality of beams respectively corresponding to four color components (yellow, magenta, cyan, and blue), a cylindrical lens 57, a single polygonal mirror 50, a scanning lens 51, and a plurality of compensation lenses 52 respectively provided for the plurality of beams.
The beams emitted by the light source 56 are converged by the cylindrical lens 57 to a point on a reflective surface of the polygonal mirror 50. As shown in FIG. 11, the beams emerged from the cylindrical lens 57 are incident on the polygonal mirror 50 with incident angles of the beams with respect to the reflective surface in the auxiliary scanning direction being different from each other.
The polygonal mirror 50 simultaneously deflects the beams incident thereon to scan in the main scanning direction. The beams deflected by the polygonal mirror 50 proceed in directions different from each other in the auxiliary scanning direction and pass through the scanning lens 51.
Each beam emerged from the scanning lens 51 is bended by a corresponding mirror group consisting of a pair of mirrors 53 and 54 or consisting of a mirror 54, and passes through the corresponding-compensations lens 52. The beams emerged from the compensation lenses 52 impinge on photoconductive drums 59, respectively, to from latent images on the photoconductive drums 59. With this structure, latent images having respective color components are formed on the photoconductive drums 59.
If one of lens surfaces of the compensation lenses 52 causes a ghost beam, it is required to decenter the lens surface causing the ghost beam or the compensation lens 52 in order to prevent the occurrence of the ghost beam. Although the occurrence of the ghost beam is prevented by decentering the lens surface of the compensation lens 52 or decentering the compensation lens 52, it is impossible to decenter the scanning lens 51 to prevent a bow caused by the decentering of the lens surface of the compensation lens 52 or the decentering of the compensation lens 52. The reason is that if the scanning lens 51 is decentered to compensate for the bow cased by one of beams passing through the scanning lens 51, the decentering of the scanning lens 51 affects the other beams which do not cause the ghost beam and therefore performance of the tandem-type scanning optical system 110 is deteriorated.
In a case where one lens (hereafter, a first lens) is decentered to compensate for a bow caused by the decentering of a lens surface of another lens, the decentering amount of the first lens required to compensate for the bow depends on the shapes of lens surfaces of the first lens. Therefore, there may be a case where a decentering amount of the first lens required to compensate for the bow becomes too great and thereby manufacturing process of the first lens becomes very difficult. In addition, there may be a case where the bow caused by the decentering of the lens surface of another lens can not be sufficiently compensated by the decentering of the first lens.